Synthesis of Phenanthridine Derivatives Functionalized in the C-Ring by Means of IMDAF Reactions under Microwave or Conventional Heating Conditions

Phenanthridines functionalized in the C-ring have been synthesized by IMDAF (intramolecular Diels-Alder reaction on furans) cyclization of ortho-furyl(alkenylamino)arenes under both microwave and conventional heating conditions. The outcomes of the reactions are highly dependent on the substitution patterns both on the dienophile and on the arene components. 5,6-Dihydrophenanthridines, which can easily be oxidized to fully aromatic phenanthridines, are formed under microwave conditions as long as the Diels-Alder adduct contains a hydrogen atom in the 6a-position. Aromatization of the phenanthridine C-ring does not occur when the cyclization is performed under conventional heating conditions. The IMDAF reaction can, under these conditions, lead to Diels-Alder adducts or to phenanthridines with partially saturated and functionalized C-rings, depending on the detailed structure of the starting material.